Flue gas has been contacted with a washing liquid containing free calcium ions which react with sulfur dioxide and with other noxious components contained in the flue gas to form corresponding calcium compounds. Calcium is added to the washing liquid as calcium oxide, calcium hydroxide and, optionally, as calcium carbonate (in part). Precipitated insoluble calcium compounds, primarily calcium sulfate in gypsum form, are removed from the washing liquid by sedimentation in a suitable device, e.g. a double-casing heavy-to-medium gravitational separator, and subsequently dried and deposited or further processed as gypsum. By suitably conducting the procedure and by adding carboxylic acid to the washing liquid, an extraordinarily high degree of purification can be achieved. Reference in this regard is made to German Patent Application No. 2,532,373. A very high stoichiometric efficiency in relation to the charged calcium oxide is thus obtained.
According to this and other wet methods, however, there is a disadvantage that the purified flue gas is severely cooled during purification. In fact, the resulting purified gas is ordinarily at a temperature of only about 55.degree. C. At such a low temperature the flue gas is not adequately disseminated in the atmosphere. Moreover, it is below the dew point of acids retained in the purified gas, thus subjecting various ducts and chimneys (through which purified flue gas is conducted) to severe damage by corrosion.